A popular drug used to regrow hair and thicken thinning locks in both men and women may be difficult to find in some pharmacies, according to new research.

The pill, called minoxidil, must be taken every day for patients to maintain the progress they’ve made in restoring hair growth. An interruption could be devastating.

“You can go backwards and start losing your hair again,” said Dr. Adam Friedman, professor and chair of dermatology at George Washington University.

Minoxidil is often used for androgenetic alopecia, also known as male or female pattern hair loss. It’s a condition that affects nearly half of men and about a quarter of women by age 50, according to the American Osteopathic College of Dermatology.

In recent months, Friedman noticed that his patients were having difficulty obtaining 30-day supplies of the drug from their Washington, D.C.-area pharmacies.

Earlier this month, he and colleagues called 277 pharmacies in the District of Columbia, Maryland and Virginia to ask about their minoxidil supplies. They found considerable shortages.

Just 40% of those pharmacies had the ability to immediately fill 30-day prescriptions for minoxidil in doses used to treat hair loss (2.5 milligrams). The research has been published in the Journal of Drugs in Dermatology.

Friedman could not identify a reason for the shortage, but said that the drug had been gaining attention through both mainstream and social media — which could have triggered a rise in prescriptions.